Dependable communication synthesis for distributed embedded systems

1. Dependable communication synthesis for distributed embedded systems Nagarajan Kandasamy, John P. Hayes, Brian T. Murray Presented by John David Eriksen and Jamie Unger-Fink

2. Overview • Real-time distributed systems • Network • Sensors • Processors • Actuators • Requirements • Strict performance targets • Fault-tolerance • Safety-critical operation

3. Related Work • Network Protocols • Controller Area Network protocol (CAN) • Common protocol used in embedded systems networks. • Messages scheduled using variable priority levels • Time-Division Multiple Access (TDMA) • Time-multiplexed frame-based protocol • Network topology considerations • Fixed network topology assumption • Synthesize topology given application requirements • Can be accomplished using a task graph model • Can be accomplished assuming a CAN or TDMA network protocol

4. TDMA Overview • Time-Division Multiple Access (TDMA) communication protocol • Faster than CAN • Used in this paper TTP and FlexRay networking protocols

5. TDMA in FlexRay A round is a set of identical-sized slots determined by a given communication schedule. A processor Pj is allocated one or more sending slots during a round. Number of slots and size of slots determined by designer. A processor can only place messages in the slots allocated to it.

6. Topology Overview Topology restricted to multiple-bus systems Multiple-buses used for fault tolerance and to spread larger communication loads – network media typically low-bandwidth and inexpensive. Each processor connects to subset of communication buses Coprocessor attached to each processor handles message communication without interrupting process execution

7. Summary of Approach Target: a low-cost and reliable communication network Use a set of distributed applications modeled as task graphs {Gi}. Allocated messages to minimum number of buses {Bj} where each Bj has a fixed bandwidth.

8. Summary of Approach Target a generic TDMA protocol (FlexRay) Assume a multi-rate system where each graph Gi may have a different execution period period(Gi) Support dependable message communication by establishing redundant transmission paths between between processors. Maintain efficient network usage by reusing transmission slots allotted to a given processor between multiple messages sent by it.

9. Topology Development • How is final number of necessary buses determined? • Initially, each message is assigned its own bus. • Iterative clustering heuristic used to minimize the number of buses used. • Clustering is NP-complete • Clustering heuristic SYNTH({mi}) used instead

10. Topology Development

11. Message Clustering • SYNTH({mi}) • Cluster synthesis • Group message mi with existing cluster Cj if: • No two replicas of one k-FT message is allocated to Cj • All messages in Cj continue to meet deadlines. • Length of Cj communication schedule does not exceed application specifications. • Memory size of Cj communication schedule must fit in memory size of communication network co-processor. • This grouping process is considered to be a bin-packing problem (NP-Complete) so a heuristic used. • Results in efficient use of bus bandwidth through sharing and re-use of transmission slots between multiple messages whenever possible. • Each cluster assigned to separate bus in final topology.

12. Task Deadline Assignment • Task scheduling range [ri, di] • ri – Release (start) time • di – Deadline • Deadline computation • NP-Complete • Hueristic used instead (Natale and Stankovic)

13. Task Scheduling Initial task graph with execution times parenthesized and fixed execution period of 2000 microseconds. Path selection and slack allocation. Path {T1,T2,T4,T5} scheduled and removed. {T3} then scheduled. Final release/deadline scheduling range.

14. Task Scheduling • Tasks assigned priorities according to the parameters of their schedule (release time and deadline) • A processor executes the highest-priority task that has been released for execution • Feasible only if: • All tasks can complete by their deadlines • All messages can be received by their deadlines

15. Case Study • Applications modeled • Adaptive cruise control - Maintain safe following distance behind moving car • Electric Power Steering - Provide steering assistance • Traction Control - Maintain intended path on slippery terrain

16. Case Study Parameters Bus bandwidth of 250 KB/s Transmission slot width of 50 microsec. Max number of slots 16 Slot reuse on or off Round lengths fixed at three different sizes: 12, 16, and 10. Round lengths are harmonic multiples of three base periods: 3, 4, and 5.

17. Case Study Results No slot reuse yielded designs with less free slots that can be used for non-critical messages. Slot reuse yielded more free slots. Optimal solution used a period base 4 and yielded solution with three buses and plenty of free slots.

18. Conclusion • Synthesis of low-cost TDMA communication network for distributed system showed to be feasible. • Proven via formal mathematical proofs. • Demonstrated via realistic case study. • Future work: • Message scheduler on co-processors not part of this design. • Fault-tolerant allocation of tasks to processors not addressed.